Fiber liberating and bleaching process



Patented Nov. 1 1, 1930 UNITED STATES PATENT OFFICE GEORGE A. RICHTER AND MILTON O. SCHUB, OF BERLIN, NEW HAMPSHIRE, ASSIGNOBS TO BROWN COMPANY, OF BERLIN, NEW HAMPSHIRE, A CORPORATION OF MAIN E FIBER LIBERATING AND BLEACHING PROCESS Ho Drawing. Application filed April 19,

This invention relates to the liberation and bleaching of cellulose fiber, and has for its object the production of fiber suitable for manufacture into papers of high pop test,

5 whiteness, and color stability.

After cellulose fiber is liberated from raw cellulosic material, such as wood chips, it is washed, screened, and, if intended for manufacture into writing papers or the like, finally bleached. During such bleaching, oxycelluloses are produced in the fiber by degradation of its cellulose content,increase in whiteness being accompanied by a lowering of the alpha cellulose content of the fiber and injury to its paper-making characteristics. This is especially true in the case of a strong sulphite fiber produced by under-digesting in a liquor of high combined S0 content,-for instance, in a liquor having a combined SO content considerabl above that of the usual calcium bisulphite liquor. The resulting fiber, while of high strength, is nevertheless difliculty bleachable, and if it be attempted to bleach such fiber to whiteness, its

'26 papermaking qualities are severely injured.

In accordance with the process of our invention, the bleaching of fiber, such for instance as that liberated in liquor of high combined SO content, is effected in a manner to preserve its papermaking characteristics. Such bleaching, we carry out in two separate steps, the first of which consists in treating the unbleached fiber with chlorine water of a strength insuflicient to whiten it, for if the fiber were so whitened, it would be injured in papermaking characteristics. The chlorinetreated fiber is then whitened in a distinctly alkaline bleach liquor, in which the fiber may 40 be brought to substantially ultimate whiteness without material degradation of its cellulose content or injury to its papermaking characteristics. Apparently the free alkali in the bleach liquor increases the selectivity of oxidation upon the ligneous and coloring 1927. Serial N0. 184,879.

materials, which are more susceptible to oxidation than is the cellulose.

An example of procedure which may be followed in producing white fiber of high strength and stability may be substantially as follows. Chipped wood,spruce, for example,is digested under pressure at an elevated temperature in a sodmm bisulphite liquor containing about 4% combined and 4% free S0 until complete fiber liberation has been elfected. Di estion in a liquor of such high combined S6 results in a rapid penetration of the combined SO, into the chips, and results in a complete fiber liberation in a much shorter time than when the usual calcium base sulphite liquor containing about 1% combined S0 and which is of much lower penetrability, is employed. The resulting fiber yield is higher and the fiber is of considerably greater strength than in the case of the usual sulphite pulp. This is probably ascribable to the fact that the fibers are longer and contain a larger proportion of the pentosan and ligneous content of the wood, as a result of lower hydrogen ion concentration in the liquor and less hydrolysis of fiber. Thus, whereas the usual unbleached sulphite pulp contains about 96% cellulose, pulp produced in a liquor containing about 4% combined and 4% free SO may contain about to cellulose, depending upon the thoroughness of cook, and is high in pentosans and ligneous matter. If such a normal or undercooked pulp were bleached as ordinarily, to a color of, say, about 100, its papermaking characteristics would be severely injured. In accordance with our process, such pulp is first treated at a stock density of about 10% with chlorine water containing about 1.5% to 4% chlorine, based on dry fiber weight. The treatment may be carried out at about room temperature for about onehalf hour or so, as the chlorine is rapidly consumed by reaction upon ligneous and other coloring lnattcr, removing or rendering such matter more susceptible to subsequent oxidation. Such treatment does not wlnten the pulp, but tends to deepen its color to a distinct yellow or light orange. The treated pulp is washed and then bleached at a stock density of about 10% in a solution containing about 5% to 10% sodium or calcium hypochlorite and about to 2% caustic soda, based on dry fiber. Bleaching may be carried out at about room, or at a temperature of 100 to 110 F., until the fiber is brought to substantially ultimate whiteness. Under such conditions, a whitening of the fiber is effected without injury to its papermaking characteristics or material degradation or the cellulose into oxycelluloses. Apparently, the free alkali not only tends to increase the selectivity of oxidation upon the ligneous matter, but also to dissolve out the ligneous reaction products of the chlorine treatment. The bleached pulp is washed and treated with an anti-chlor, e. g., sodium bisulphite, to remove residual chlorine and to neutralize it.

The bleached pulp has a high pop test and yields hard, rattly papers, so that it is particularly suitable for the manufacture of glassine papers and bond papers of high color, stability, and strength. Thus, papers formed from pulp liberated and bleached as 'hereinbefore described have a pop test of about 140 to 160, as compared with a pop test of about 115 to 125 of the usual sulphite pulp papers.

As hereinbefore stated, one of the advantages of our process is that wood pulp produced by digestion in a liquor having a high combined S0 content may be whitened without materially injuring its papermaking characteristics. Such a process is economically advantageous, as a high yield of pulp is produced by digestion, which, in turn, is

reflected in a higher yield of bleached pulp,

such latter yield amounting to about based on wood, as compared with a yield of about 45% for the usual sulphite pulp. .The bleached pulp has a high pop strength, and at the same time a tear resistancecomparable to the usual sulphite pulp, and although of high color, possesses remarkable color stability and is low in copper number.

While we have described a specific example of procedure wherein pulp is liberated by digesting wood chips in a cooking liquor having a combined and free S0 content of about 4% each, it should be understood that pulp liberated in other liquors, for instance, in the usual calcium bisulphite liquor or in sodium bisulphite or in mixed sodium bisulphite and calcium bisulphite liquors containing 2% combined and 5% free S0 or other proportions of combined and free, may be advanta geously bleached as hereinbefore described. \Vhere liquors containing high combined S0 contents are employed, the chips are preferably undercooked to produce hard, strong ulps, as our bleaching treatment is applicalile to greatest advantage in such case.

The bleaching process herein described may be applied successfully to unbleached pulps otherthan sulphite, such for instance as well-cooked soda or kraft pulps, particularly when such latter pulps are liberated from such wood as birch, beech, maple, and poplar. It finds great utility, however, in the bleaching of hard, strong sulphite pulps to produce a high yield of strong, white fiber of high stability, as indicated by the specific example herein given.

Having thus described certain examples of procedures, it should be understood that these procedures might be deviated, from without departing from the spirit or scope of invention as defined by the appended claims.

' What we claim is:

1. A process which comprises treating unbleached pulp with chlorine water, and then bleaching such pulp in a bleach liquor maintained distinctly alkaline throughout such bleaching and containing suflicient bleach to whiten such pulp.

2. A process which comprises digesting wood chips in a sulphite liquor having a combined SO content considerably above thatof the usual calcium bisulphite liquor, treating the resulting pulp with chlorine water, and bleaching the treated pulp in a distinctly alkaline bleach liquor containing sufiicient bleach to whiten such pulp.

3. A process which comprises digesting wood chips in a sulphite liquor having a combined S0 content considerably above that of the usual calcium bisulphite liquor and a free S0 content about equal to its combined SO content, treating the resulting pulp with chlorine water but insufliciently to whiten it, and bleaching such pulp in a distinctly alkaline bleach liquor containing sufiicient bleach to whiten such pulp.

4. A process which comprises digesting wood chips in a sodium bisulphite liquor having a combined and free S0 content of about 4% each, treating the resulting pulp With chlorine water but insufliciently to whiten it, and bleaching such pulp in a distinctly alkaline liquor containing suflicient bleach to whiten such pulp.

5. A process which comprises undercooking wood chips in a sulphite liquor having a combined SO content considerably above that of the usual calcium bisulphite liquor, treating the resulting pulp with chlorine Water of a strength which results in a deepening of the color of such pulp, and bleaching the treated pulp in a distinctly alkaline bleach liquor containing suflicient bleach to whiten such pulp.

6. A process which comprises digesting wood chips in a sodium bisulphite liquor having a combined and free SO, content of about 4% each, treating the resulting pul with chlorine water but insufliciently to w iten it, and bleaching such pul in a hypochlorite 5 bleach liquor maintame distinctly alkaline with caustic soda throughout such bleaching and containing sufi'lcient bleach to whiten such pulp.

In testimony whereof we have aflixed our signatures.

GEORGE A. RICHTER MILTON O. SCHUR. 

